1. Field of the Invention
This invention relates to an improved process for concentrating aqueous urea solutions and, more particularly, to improvements in a two-step process for concentrating aqueous urea solution by use of a stream of hot inert gas.
2. Description of the Prior Art
An aqueous urea solution obtained by reacting ammonia and carbon dioxide at high temperature and pressure and subjecting the resulting urea synthesis effluent to conventional procedure for the removal of unreacted materials has a concentration of 70-90% by weight. Accordingly, in order to prepare granulated urea which is a common form of urea for practical use, this aqueous urea solution need be concentrated to not less than 99.5% by weight. There have been proposed a number of processes for effecting such concentration. As an example, U.S. Pat. No. 3,491,821 discloses a two-step process for concentrating aqueous urea solutions. More specifically, in the first step of this process, an aqueous urea solution having a concentration of 65-95% by weight is allowed to flow as a falling film along the interior surface of externally heated tubes and brought into countercurrent contact with a stream of hot inert gas, whereby it is concentrated to 97.5-99% by weight. In the succeeding second step, the aqueous urea solution resulting from the first step is passed through a packed zone in countercurrent contact with a stream of hot inert gas. (The stream of hot inert gas leaving the packed zone serves as the stream of hot inert gas used in the first step.)
However, in the process of U.S. Pat. No. 3,491,821 wherein an aqueous urea solution is brought into countercurrent contact with a stream of hot inert gas in the second step, blowing-up of the solution by the stream or back mixing of the solution due to blowing-up of its droplets by the stream occurs. Thus, the aqueous urea solution to be concentrated departs from the state of piston flow, so that the evaporation rate is decreased. (Supposing that this back mixing causes the aqueous urea solution passing through the concentrator to be in the state of complete mixing, the concentrator must be operated in such a way that the composition of the solution within the concentrator is equal to that of the solution at its outlet. Accordingly, the capacity for evaporating water from the solution will be minimized. Actually, the aqueous urea solution is presumed to be in an intermediate state between piston flow and complete mixing.) The influence of this back mixing becomes more marked in the concentration range of about 99% by weight and greater where the vapor pressure of water above the aqueous urea solution is sharply reduced as the concentration increases. Such a decrease in evaporation rate naturally prolongs the residence time in the apparatus (or packed zone). Consequently, the conversion of urea into biuret cannot be avoided and the size of the apparatus (or packed zone) need be increased in proportion to the prolonged residence time.